Oxidation catalyst

ABSTRACT

A class of ceramic mixed oxide, nonstoichiometric electrically neutral rare-earth-type catalyst containing rare-earth-type elements and elements of the first transition metal series and optionally the alkaline earth metals. The catalyst has the following formula: 
     
          X.sub.n Y.sub.(1.sub.-n) ZO.sub.(3&lt;m)                     (I) 
    
     wherein: 
     X is an alkaline earth metal or mixture thereof; 
     Y is a rare-earth-type element or mixture thereof; 
     Z is a metal of the first transition series or a mixture thereof, at least 0.01% of said metal having an oxidation state other than +3; 
     m is a number having a value of between 0 and about 0.11; and 
     n is a number having a value from 0 to about 0.51. 
     These mixed oxide catalysts can be used to catalytically oxidize low molecular weight inorganic compounds and elements, such as ammonia, carbon monoxide, hydrogen, sulfur dioxide, and hydrogen sulfide, with oxygen, or carbon monoxide with water, sulfur dioxide or nitric oxide. The catalyst can also be employed in the catalytic removal of carbon monoxide, hydrocarbons and nitric oxides from the exhaust gases of generating or heating plants and automobiles burning fossil fuels.

This is a division of Ser. No. 99,239, filed Dec. 17, 1970.

This invention is directed to a class of oxidation catalysts, tocatalytic oxidation processes utilizing such catalysts, and to methodsof catalytically treating exhaust gases with such catalysts to produceexhaust gases substantially free of harmful pollutants.

More particularly, the present invention is directed to a class ofceramic mixed oxide nonstoichiometric electrically neutral oxidationcatalysts of the following formula:

    X.sub.n Y.sub.(1.sub.-n) ZO.sub.(3.sub.+-m)                (I)

wherein:

X is an alkaline earth metal or mixture thereof;

Y is a rare-earth-type element or mixture thereof;

Z is a metal of the first transition series or a mixture thereof, atleast 0.01% of said metal having an oxidation state other than +3;

m is a number having a value of between 0 and about 0.11; and

n is a number having a value from 0 to about 0.51.

Further, the present invention is directed to processes for thecatalytic oxidation of ammonia, carbon monoxide, hydrogen, hydrocarbons,sulfur dioxide and hydrogen sulfide with oxygen; of carbon monoxide withwater vapor, sulfur dioxide or nitric oxide; and of a gaseous mixture ofuncombusted or partially combusted fossil fuel, carbon monoxide, carbondioxide and atmospheric gases with oxygen without the concomitantproduction and emission of nitric oxide in the exhaust gas, employingthe catalyst described herein. Furthermore, the present invention isdirected to methods of treating exhaust gases from chemical plants,electrical utility generating plants, heating plants, steel mills,smelting plants, trucks and automobiles to remove gaseous pollutants,such as carbon monoxide, hydrocarbons, partially oxidized hydrocarbons,the oxides of nitrogen, and sulfur dioxide, therefrom.

Since the advent of modern technology, air pollution has become aserious and sometimes tragic problem for man. The present industrialplants and automobiles which burn the fossil fuels emit a staggeringamount of gaseous pollutants, principally unburned or partially burnedfossil fuels, carbon monoxide, the oxides of nitrogen, sulfur dioxideand ozone. These pollutants are chemically reactive and have been foundto be harmful to both plant life and animal life. Under certain weatherconditions, the accumulative emissions of these pollutants can havetragic effects; for example, in late 1930 about sixty people died andanother 6,000 became seriously ill from breathing abnormally high levelsof gaseous pollutants in the Muse Ruhr Valley of Belgium. A more recentcalamity took place in the mill town of Donora, Pennsylvania, in 1948.The death smogs or fogs that hovered over London, England, in 1952 and1962 are well documented. As stated above, gaseous pollutants not onlyaffect animal life but they also affect plant life. Smog has been foundto have a serious effect on the evergreen trees growing on the mountainsides surrounding the Los Angeles basin. Furthermore, the University ofCalifornia Air Pollution Research Center has found that smog has adetrimental effect on the growth and the fruit yield of citrus trees.

Smog is a by-product of a complex series of photosynthetic reactionsthat occur in the atmosphere when certain molecular species are foundtherein. More particularly it has been found that the production ofphotochemical smog requires nitrogen oxides such as nitric oxide andnitrogen dioxide, hydrocarbons, and ultra-violet light. One of thepossible routes for the production of smog is theorized as follows:During the combustion of a fossil fuel the oxides of nitrogen areformed. These nitrogen oxides and some unburned fuel together withcarbon monoxide are emitted in the exhaust gases. In the presence ofhydrocarbons and sunlight the nitric oxide is photochemically oxidizedto nitrogen dioxide. The nitrogen dioxide mole is then photochemicallysplit into nitric oxide and atomic oxygen. A portion of the atomicoxygen in turn interacts with molecular oxygen to form ozone or withhydrocarbons to form complex and reactive oxidation products. It thenappears that a portion of the ozone reacts with the nitric oxide toprovide a fresh supply of nitrogen dioxide. Another portion of theatomic oxygen and a portion of the reactive hydrocarbon oxidationproducts in the air form free radicals which in turn react very readilywith oxygen, nitric oxide, nitrogen dioxide, and with other hydrocarbonsto form more complex materials such as peroxyacyl nitrates. Theperoxyacyl nitrates are suspected of being the principal cause of theeye-searing effect of smog; it has been documented that these compoundscan cause substantial damage to crops when present in exceedingly smallamounts, such as parts per hundred million.

From the time the primary gaseous pollutants known to causephotochemical smog were identified, it has been recognized that theelimination of these gaseous pollutants from the exhaust of industrialplants and vehicles would substantially eliminate photochemical smog.The chemical industry has been working diligently in this field over thelast decade in an effort to accomplish this result. Some of the efforthas resulted in the introduction of devices that give limited results,such as the smog-emission devices introduced on automobiles in the late1960's. Other efforts have been directed toward catalysts and catalyticsystems which can reduce the amount of hydrocarbons, carbon monoxide andnitric oxide emitted from the exhaust gas of motor vehicles andindustrial plants burning fossil fuels. This effort has only beenpartially successful for a variety of reasons. For example, many of thecatalysts developed were prepared from precious metals such as platinumand palladium; the resulting catalysts are expensive and are noteconomically feasible to use. Other catalysts have been developed whichare readily deactivated by the presence of sulfur, oxygenated sulfurcompounds or metals, such as lead. Other catalysts, that have beendeveloped, are environmentally and/or chemically sensitive and arerapidly inactivated when operated at high temperatures or in thepresence of certain materials. Many of the present catalysts are onlyeffective at low space velocities and can only be utilized in acatalytic system having a large catalytic bed and catalytic chamber toprovide sufficient contact time between the exhaust gases and thecatalyst. The catalytic systems that have been developed are generallyquite complex and require more than one catalyst with each catalystoperating within a particular temperature range. To our knowledge nocatalytic system has been developed for the removal of gas pollutantsfrom fossil fuel exhaust fumes utilizing one catalyst which caneffectively operate over a broad temperature of the exhaust gases.

An object of the present invention is to provide an oxidation catalystwhich exhibits excellent efficiencies over a broad temperature range andat high space velocities. More particularly, it is an object to providean oxidation catalyst that exhibits excellent chemical and thermalstability.

Another object of the present invention is to provide an oxidationcatalyst which is effective in selectively oxidizing a broad range ofmolecular species, such as ammonia, carbon monoxide, hydrocarbons, andthe like. More particularly, it is an object to provide an oxidationcatalyst that can be used in the treatment of exhaust gases fromindustrial plants and motorized vehicles utilizing fossil fuels for theremoval of substantially all carbon monoxide, combusted and partiallycombusted hydrocarbons, oxygenated hydrocarbons and oxides of nitrogentherefrom.

Ceramic, stoichiometric compounds of the following empirical formula areknown and have been used in high temperature electrodes:

    X'.sub.n Y'.sub.(1.sub.-n) Z'O.sub.3

wherein:

X' is strontium,

Y' is yttrium or lanthanum,

Z' is a metal of the first transition series, and

n is 0 or 0.22.

The catalytic activity of these compounds has apparently never beenrecognized and we believe we are the first to discover their catalyticproperties. In an attempt to improve the catalytic activity of the aboveceramic compounds we made the discovery that we could prepare ceramic,nonstoichiometric, electrically neutral compounds of the followingformula and that in most cases these compounds exhibited superiorcatalytic activity because of their nonstoichiometric nature:

    X.sub.n Y.sub.(1.sub.-n) ZO.sub.(3.sub.+-m)                (I)

wherein:

X is an alkaline earth metal or mixture thereof;

Y is a rare-earth-type element or mixture thereof;

Z is a metal of the first transition series or a mixture thereof, atleast 0.01% of said metal having an oxidation state other than +3;

m is a number having a value of between 0 and about 0.11; and

n is a number having a value from 0 to about 0.51.

The catalyst of the present invention is a ceramic, mixed oxide,nonstoichiometric electrically neutral catalyst containing arare-earth-type element or mixture thereof, a metal of the firsttransition series or mixtures thereof, oxygen atoms, and optionally analkaline earth metal or mixture thereof. The transition metal or metalsare present as mixed oxides wherein the metal is present in more thanone oxidation state. The rare-earth-type element or elements can also bepresent in more than one oxidation state.

The alkaline earth metals include beryllium, magnesium, calcium,strontium, barium, and radium; in the present invention the preferredalkaline earth elements are magnesium, calcium, strontium, and barium.The rare-earth-type elements are scandium, yttrium and the rare earthelements, e.g., lanthanum, cerium, praseodymium, neodymium, promethium,samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,thulium, ytterbium, and lutetium. In the present invention the preferredrare-earth-type elements have atomic numbers between 20 and 72. Themetals of the first transition series include titanium, vanadium,chromium, manganese, iron, cobalt, nickel, copper and zinc. In thepresent invention the preferred metals of the first transition seriesinclude those metals having an atomic number between 21 and 31. At least0.01% of the transition metal atoms of the oxide catalyst have oxidationstates other than +3; i.e., at least 0.01% of the transition metal atomsare present in oxidation states higher or lower than +3. In thepreferred embodiment of the present invention, at least 0.1% of thetransition metal atoms in a mixture of metal oxides have oxidationstates other than +3. Although there is no upper limit to the percentageof metal atoms having oxidation states other than +3, rarely will morethan 35% of the transition metal atoms have oxidation states deviatingfrom +3. The rare-earth-type elements may be present in more than oneoxidation state, i.e. between and including +2 to +4.

The ceramic, mixed oxide, nonstoichiometric, electrically neutralcatalyst of the present invention is a solid crystalline compound at thetemperatures at which it is used. Elements represented by X, Y and Z offormula (I) are present in the catalyst as positively charged ions andthe oxygen is present as negatively charged ions. The chemical bondingbetween X, Y, Z and O is most accurately described as being ionic. As asolid chemical compound the catalyst is partially characterized, webelieve, by its crystal structure. It differs from the crystalstructures of catalysts described as mixtures of oxides such as thehopcalites which are mixtures of metal oxides and not a homogenous mixedoxide compound.

Electrical neutrality in the mixed oxide catalyst of the presentinvention can be understood in terms of classical theory regarding ionicspecies and present theories regarding defect chemistry. Electricalneutrality in the present mixed oxide catalyst, in which fractions ofthe elements represented by Z and Y are present as positive ions withcharges other than +3, is maintained by a balance of negatively chargedand positively charged ionic species and the presence of defects such aspositively charged holes, interstitial positive ions, positive ionvacancies, negatively charged electrons and oxygen vacancies. An oxygenvacancy is an empty oxygen lattice site in a crytal structure normallyoccupied by O.sup.⁻² ions. The defects are particularly important inbalancing the X and Z elements having charges other than +3. A fractionof the oxygen vacancy defects may exist as oxygen vacancy defectcomplexes in which the oxygen vacancy is localized preferentially abouta particular positively charged ion. The catalyst exhibits highelectronic conductivity at temperatures at which the defects, i.e.holes, vacancies, or electrons are mobile. Ionic conductivity within thecatalyst is associated with the mobility of the oxygen ions and isenhanced by the presence of defects such as oxygen vacancies or oxygenvacancy complexes.

It appears that the surprising activity of the present catalyst arisesfrom the nonstoichiometric character of the metal oxide compositioncontained therein. The present catalyst has a very high electronicconductivity and ionic conductivity. The sum of the electronicconductivity and the ionic conductivity is the electrical conductivityof material and is inversely proportional to the electrical resistanceof the material. The present catalyst has an electrical conductivity ofabout 0.1 ohms ^(-1cm)..sup.⁻¹ to about 1000 ohms .sup.⁻¹ cm..sup.⁻¹.The ionic conductivity is a measurement of the ion flow or migrationthrough and over a material. Most materials found in nature have verylow ionic conductivities; however, molten salts and salts dissolved inwater have very high ionic conductivities. The present catalyst has ahigh ionic conductivity and will readily permit ions to flow or migratethrough and across its surfaces for the distances of 100 angstroms ormore. The present ceramic, nonstoichiometric oxidation catalysts alsohave excellent selective adsorption properties. The present catalystsare relativelyl good adsorbers of partially oxidized or partiallyreduced molecular species such as carbon monoxide, sulfur dioxide, theoxides of nitrogen, and hydrogen sulfide, and they are relatively pooradsorbers of fully oxidized or reduced materials, such as carbon dioxideor nitrogen gas. The adsorption qualities of the present catalyst alsoappear to arise from its nonstoichiometric nature.

Without intending to limit the invention by the following discussion, itappears that many of the surprising characteristics of the presentcatalyst can be attributed to its high ionic and electronicconductivity, and selective adsorption characteristics. Under presenttheory, an effective catalyst must be a good adsorber of reactants and arelatively poor adsorber of the reaction products thereof. If thecatalyst is a relatively poor adsorber of reactants, the space velocityof the gas stream containing the reactants and the relative gas pathwaylength throughout the catalyst are usually adjusted to give thereactants and catalytic surface the optimum contact conditions. Therelatively high electronic conductivity of the present catalyst due toeither mobile holes or electrons tends to enhance the effective areas ofthe catalyst by reducing the areas of the space charge regions aroundadsorbed molecules which tend to become polarized or ionic. Underpresent theory, it also appears that if the given reaction is a Red-Oxreaction, that is a reaction involving the oxidation of one reactant andthe reduction of the other reactant, the site of reduction and the siteof oxidation on the catalytic material must be relatively close, such asabout 10 A or less, so that electrons and/or molecular ionic speciesresulting from the partial reduction or oxidation of the reactants canmigrate between the two sites to complete the reaction. For example,when the catalyst is to be employed in the oxidation of carbon monoxidewith water vapor to form carbon dioxide and hydrogen gas, the carbonmonoxide molecule will become attached to an oxidation site and becomechemically excited wherein it will readily accept either monoatomicoxygen or ionic oxygen to form carbon dioxide. A water molecule willbecome attached to a reduction site wherein the water molecule will besplit apart to form diatomic hydrogen and monoatomic or ionic oxygen.The former will escape from the catalyst surface while the latter willmigrate over the catalytic surface or through the catalyst as oxygenions to a nearby carbon monoxide oxidation site to combine therewith andform carbon dioxide which will subsequently escape from the surface ofthe catalyst. If the oxidation and reduction sites are more than 10 Aapart, the catalyst generally will not be very effective and willexhibit little catalytic activity. In the present catalyst, therespective reaction site can be separated by distances far exceeding 10A; for example, the reaction sites can be separated by distances of 100A or more, because the catalyst has high ionic and electronicconductivity which provides excellent mobility for electrons and atomicand molecular ionic species. Consequently, in the present catalystelectrons or holes can readily flow and migrate from site to site duringa given chemical reaction and likewise oxygen ions can migrate and flowfrom site to site with relative ease.

The present catalysts are prepared by making up an aqueous solution ofthe corresponding water-soluble salts of the rare-earth-type elements,metals of the first transition series and alkaline earth metals, if thelatter are to be included. Typical water-soluble salts that are employedinclude the nitrate and halide salts of the rare-earth-type elements,the metals of the first transition series and the alkaline earth metals.After the aqueous solution is thoroughly mixed, the solution isevaporated at either room temperature or at elevated temperatures todryness and the resulting residue is calcined for several hours atelevated temperatures, such as temperatures between 900° C. and 1500° C.If the salts lack oxygen atoms, the salt mixture is calcined in thepresence of oxygen gas. The resulting ceramic nonstoichiometricoxidation catalyst is a fine powder which can be readily milled, pressedand sintered to any desired shape. Alternatively, the powdered catalystcan be moistened, molded or extruded into a desired shape, and thensintered or fired. Alternatively, the present catalyst can be preparedfrom the water-insoluble salts, such as the sulfates, carbonates, oroxides of the rare-earth-type elements, the metals of the firsttransition series, and the alkaline earth metals. The salts areparticulated and thoroughly mixed in their appropriate molar amounts.The resulting mixture of salts is then calcined as described above toprepare the ceramic oxidation catalyst of the present invention.

Referring to the above formula (I), the value of (3±m) is affected bythe oxygen pressure during the calcining step. If the catalytic startingmaterial is calcined in the absence of oxygen or at low oxygen partialpressures such as 10 mm Hg, the value of the quantity (3±m) will besmaller than if the starting catalytic material is calcined at highoxygen pressures such as 700 mm Hg, 3 atmospheres or the like.

As described above, the present catalyst of formula (I) and the ceramic,mixed oxide, stoichiometric, electrically neutral catalyst of thefollowing formula

    X.sub.n Y.sub.(1.sub.-n) ZO.sub.3                          (II)

wherein X, Y, Z, O and n are as defined above, can be employed in theoxidation of a wide variety of molecular species. For example, the abovecatalysts can be utilized in the following oxidation reactions:

    A.  2CO + O.sub.2 →2CO.sub.2                                           B.  2H.sub.2 + O.sub.2 →2 H.sub.2 O                                    C.  Hydrocarbons and                                                              Oxygenated Hydro-                                                             carbons such as                                                               Aldehydes,                                                                    Ketones,       + nO.sub.2 →mCO.sub.2 + pH.sub.2 O                      Ethers,                                                                       Alcohols, and  wherein n, m and p are                                         Carboxylic acids                                                                             integers having a value                                                       of 1 or more.                                              D.  2SO.sub.2 + O.sub.2 →2SO.sub.3                                     E.  2H.sub.2 S +3O.sub.2 →2SO.sub.2 + 2H.sub.2 O                       F.  4NH.sub.3 + 40.sub.2 →2N.sub.2 O + 6H.sub.2 O                      G.  4NH.sub.3 + 50.sub.2 →4NO + 6H.sub.2 O                             H.  2NO + O.sub.2 →2NO.sub.2                                           I.  2CO + 2NO→2CO.sub.2 + N.sub.2                                      J.  CO + H.sub.2 O→CO.sub.2 + H.sub.2                                  K.  4CO + 2SO.sub.2 →4CO.sub.2 + S.sub.2                               L.  4H.sub.2 S + 2SO.sub.2 →3S.sub. 2 + 4H.sub.2 O                     M.  Hydrocarbons + CO + CO.sub.2 + H.sub.2 O + N.sub.2 + O.sub.2 (slight            excess)→CO.sub.2 + H.sub.2 O + N.sub.2 + O.sub.2 (residue)   

Reactions A-H and M normally will be carried out with atmosphericoxygen, i.e., with air. In such cases, in each of these oxidationreactions there will be little concomitant oxidation of the atmosphericnitrogen to the nitrogen oxides such as a nitric oxide. However,reactions A-H and M can be carried out with any oxygen containing gas.Reaction A can be conducted with only a trace of carbon monoxide andrequires only a slight excess of the stoichiometric amount of oxygenneeded for the complete combustion oxidation of the carbon monoxide tocarbon dioxide. The reactants, i.e., the carbon monoxide and oxygen, arepassed through a bed of the catalyst at a temperature between about 100°C. and about 1000° C. Reaction B can be conducted with only a traceamount of hydrogen and requires only a slight excess of a stoichiometricamount of oxygen needed for the oxidation of the hydrogen. In thisreaction the reactants are passed through a bed of the catalyst at atemperature between about 100° C. and about 500°. Reaction C can bepracticed with hydrocarbons, such as methane, butane, benzene, or thelike, aldehydes such as acetaldehyde or hexanal, ketones such as acetoneor diacetone alcohol, alcohols such as methyl alcohol, propyl alcohol,or the like, carboxylic acids such as acetic acid, decanoic acid, ormixtures of the above. This reaction requires an excess of thestoichiometric amount of the oxygen needed for the complete combustionof the above-described reactants. The reactants and oxygen are passedover the catalyst at temperatures between about 100° C. and about 1000°C. At higher temperatures the reaction rate is appreciably increased andpermits very high space velocities. Reaction D can be conducted withtrace amounts of sulfur dioxide and requires an excess of thestoichiometric amount of oxygen needed for the oxidation of the sulfurdioxide. The reaction is conducted through a bed of the catalyst at atemperature between about 120° C. and about 800° C. Reaction E can beconducted with a trace amount of hydrogen sulfide and requires an excessof the stoichiometric amount of oxygen needed for the oxidation of thehydrogen sulfide. The reaction is conducted by bringing the reactants incontact with the catalyst at a temperature between about 100° C. andabout 700° C. Reaction F can also be conducted with trace amounts ofammonia and requires an excess of the stoichiometric amount of oxygenneeded for the oxidation of the ammonia. The reaction is carried out bybringing the reactants in contact with the catalyst of the presentinvention at a temperature between about 100° C. and about 400° C.,preferably between about 250° C. and about 400° C. Reaction G is a veryuseful reaction and provides a method of making nitric oxide, the firststage in the manufacture of nitric acid. The reaction requires only aslight excess of the stoichiometric amount of oxygen needed to oxidizethe ammonia to nitric oxide. The reaction is conducted at temperaturesbetween about 100° C. and about 1000° C., preferably at a temperature ofabout 400° C. or higher. Reaction H represents the second stage in themanufacture of nitric acid from ammonia. The reaction is conducted attemperatures between about 100° C. and about 400° C. Reaction I is alsoa very useful reaction and it provides a route for the elimination ofboth carbon monoxide and nitric oxide from the exhaust gas of industrialplants and motorized vehicles utilizing fossil fuels. Both reactants canbe present in trace amounts and can be present in equal stoichiometricamounts. The reaction is carried out in the presence of the catalyst ata temperature between about 200° C. and about 1000° C. When the reactionis conducted in the presence of oxygen or atmospheric oxygen, reactionsA and H are favored and the principal by-products will be carbon dioxideand nitrogen dioxide. Reaction J is also a useful reaction and providesan alternative route for the elimination of carbon monoxide in exhaustgases produced by the combustion of fossil fuels. Both reactants can bepresent in trace amounts and either can be in excess. This reaction isalso useful for commercial production of hydrogen. The reaction isconducted in contact with the catalyst at a temperature between about150° C. and about 500° C. If this reaction is conducted in the presenceof atmospheric oxygen, reaction A is favored and the principal reactantsare carbon dioxide and water from the combustion of hydrogen andatmospheric oxygen. Reactions D and E provide a means of inexpensivelyproducing sulfur trioxide for sulfuric acid production from sulfurdioxide and/or hydrogen sulfide. The reaction can be conducted with pureoxygen or with oxygen enriched air.

Reaction F provides a means of inexpensively producing nitrous oxide.This reaction can be conducted with pure oxygen or with oxygen enrichedair.

Reaction M is a reaction between atmospheric oxygen and an exhaust gasfrom industrial plants or motor vehicles burning fossil fuels with anoxygen deficiency. The exhaust gas will contain hydrocarbons and/oraldehydes, ketones, alcohols, carboxylic acids, and the like, carbonmonoxide, carbon dioxide, water vapor, and atmospheric nitrogen. Thereaction is conducted with a slight excess of atmospheric oxygen, thatis a slight excess of the stoichiometric amount of oxygen needed forcompletely combusting the hydrocarbons or other like organic species andcarbon monoxide to CO₂ and H₂ O. The effluent gas will contain carbondioxide, water vapor, atmospheric nitrogen and residual oxygen and willbe substantially free of hydrocarbons, carbon monoxide, and oxids ofnitrogen, particularly nitric oxide. The exhaust gases are put incontact with the catalyst of the present invention at a temperaturebetween about 200° C. and about 700° C., preferably about 400° C.

Reactions K and L represent useful reactions for recovering SO₂ aselemental sulfur by catalytic oxidation of CO and H₂ S in the absence ofexcess air. Reaction temperatures must be sufficiently high so as toavoid the condensation of solid or liquid sulfur on the catalyst. Thereaction temperatures are typically below 800° C.

As described above, the above catalysts of formulas I and II are veryeffective oxidation catalysts for the harmful gaseous pollutants foundin the exhaust gases of industrial plants and motorized vehiclesutilizing fossil fuels. Accordingly, the catalyst can be used to providemethods of eliminating these pollutants from exhaust gases. For example,one method of eliminating or substantially reducing the nitric oxide,carbon monoxide, and hydrocarbon pollutants from the exhaust gas ofpower plants, generating plants and steam plants utilizing fossil fuelssuch as natural gas, gasoline, oil stock and/or coal will comprise thefollowing steps: burning the fossil fuel in an oxygen deficiencyenvironment at a temperature between about 1600° C. and about 850° C. toproduce an exhaust gas containing carbon dioxide, carbon monoxide, watervapor, unburned or partially burned fossil fuel and atmospheric nitrogenwith the carbon dioxide and carbon monoxide being present in the ratioof about 100 to between about 1 and about 11. The combustion of thefossil fuel produces usable heat energy which in turn is employed toproduce steam for steam turbines or expand the gases in a reciprocalengine or gas turbine. After the combustion gas has performed its energyfunction, its temperature will have dropped to between 300° C. and 700°C. The resulting cooled combustion gas will then be combined with aslight excess of atmospheric oxygen, i.e., at least a 1% excess of thestoichiometric amount of atmospheric oxygen needed to fully oxidize theremaining uncombusted or partially combusted fossil fuel and carbonmonoxide in the combustion gas. The resulting air combustion gas mixturewill then be passed through a bed of the catalyst at a temperaturebetween about 300° C. and about 700° C. to fully oxidize the carbonmonoxide and fossil fuel. The effluent gas will contain carbon dioxide,water vapor, atmospheric and residual oxygen. The effluent gas will besubstantially free of all fossil fuels or remnants thereof, carbonmonoxide and nitric oxide. When the first combustion step is conductedat 1200° C. and the subsequent catalytic treatment is conducted at 600°C. the resulting effluent gas will contain less than 1 ppm of fossilfuels or remnants thereof, less than 10 ppm of carbon monoxide and lessthan 10 ppm of nitric oxide. In sharp contrast the present industrialplants, such as steam-generating plants, which burn fossil fuel and usea single-step combustion process, wherein the fuel is burned in anexcess of oxygen, produce an exhaust gas containing 500 ppm offossilized fuel or remnants thereof, more than 250 ppm of carbonmonoxide and more than 340 ppm of nitric oxide. These pollutants, asdescribed above, are a major contributor to photochemical smog. Thesituation in motorized vehicles is even worse. Less than 10% of themotorized vehicles are properly adjusted and have their smog emissiondevices working at optimum conditions. The remainder of the vehicles arenot properly tuned, have inoperative or poorly operating smog emissiondevices and/or faulty mufflers. The motorized vehicles emit vastquantities of unburned or partially combusted gasoline, massive amountsof nitric oxygen and appreciable amounts of carbon monoxide. The presentinvention contemplates a method utilizing the above catalysts foroperating a motorized vehicle which will emit an exhaust gas which issubstantially free of fossil fuels or remnants thereof, carbon monoxideand nitric oxide. Preferably the gasoline employed in the motorizedvehicle would be lead free. However, gasolines containing less than 1gram of lead per gallon could be utilized in the method described hereinsince the catalysts are relatively insensitive to lead contamination.The reciprocal engine will be adjusted to run on a rich mixture, thatis, the engine will be run with an overall deficiency of oxygen. Theengine can be adjusted so that it runs on an oxygen deficiency ofbetween 10% and 0.1% so as to limit the formation of nitric oxide. Theexhaust gases from the combustion chambers of the engine are vented orpiped into a catalytic muffler containing the ceramic, mixed oxidecatalyst of the present invention. The exhaust gases would containunburned fuel or partially burned fuel, carbon dioxide, carbon monoxide,water vapor, and atmospheric nitrogen. Just before entering thecatalytic muffler, the combustion gases could be combined with at least1% of excess of atmospheric oxygen, that it, at least 1% in excess ofthe stoichiometric amount of oxygen needed for the complete combustionof the carbonaceous material in the combustion gas. The resultingmixture would then be passed through the catalytic muffler at atemperature between about 200° C. and about 700° C. The resultingeffluent gas would contain carbon dioxide, water vapor, atmosphericnitrogen, and residual oxygen. The effluent gas would be substantiallyfree of all fuel and remnants thereof, carbon monoxide, and nitricoxide. The above catalysts are ideally suited to the above-describedmethod for two reasons: (1) the catalysts are relatively unaffected bymetallic substances, such as lead, and (2) the present catalysts willselectively oxidize carbonaceous material, such as carbon monoxide andhydrocarbons, in preference to atmospheric nitrogen over a broad rangeof temperatures.

The present catalyst can also be used to produce a two-step catalyticprocess for removing gaseous pollutants from fossil fuel exhausts, thatis, the exhaust gases produced by the combustion of fossil fuels. Inthis method, the fossil fuel can be burned with a stoichiometric amountof atmospheric oxygen or a slight deficiency. The fossil fuel can beburned at a temperature of 1000° C. or higher. At temperatures of 1400°C. the fossil fuel is burned with a deficiency of oxygen in order thatat least as much carbon monoxide, on a molar basis, is produced asnitric oxide, preferably at least twice as much carbon monoxide isproduced; nitric oxide production is disproportionately increased athigh temperatures. The combustion gases will contain fossil fuel,partially combusted fossil fuel, carbon dioxide, carbon monoxide, watervapor, nitric oxide and atmospheric nitrogen. If fossil fuel containssulfur or sulfur containing compounds, sulfur and hydrogen sulfide willalso be found in the combustion gases. After the combustion gases havefulfilled their energy function, they will have cooled to about 700° C.or less. The cooled combustion gases are passed through a bed of thecatalyst wherein the carbon monoxide is oxidized by nitric oxide andwater to carbon dioxide. If the fuel contains sulfur containingcompounds, the resulting effluent gas may be cooled to recover thesulfur. The resulting effluent gas is then mixed with a slight excess ofatmospheric oxygen and passed through a second bed of catalyst of thepresent invention wherein the fossil fuel, partially combusted fossilfuel, remaining carbon monoxide, remaining nitric oxide, hydrogensulfide, and sulfur are fully oxidized yielding an exhaust gasconsisting essentially of carbon dioxide, water vapor, sulfur trioxide,atmospheric nitrogen, nitrogen dioxide and residual atmospheric oxygen.The exhaust gases will be essentially free of the gaseous pollutants,i.e. oxides of nitrogen and sulfur. This method is particularlyapplicable for use on motorized vehicles and high-temperature industrialplants wherein a large quantity of nitric oxide is produced during thecombustion step.

As described above, the above compounds of formulas (I) and (II) can beformed and utilized in various states and shapes such as powdered form,pellets, flakes, and the like. For the industrial plant use or catalyticmuffler use, the catalyst can be used as a powder, as pressed sinteredpellets, macaroni-shaped tubes, flat or curved flakes, and the like. Thecatalyst also can be used on a support such as a ceramic refractory,glass or high-melting metal support substance. As mentioned above, thepowdered catalyst can be pressed into shape and fired or sintered to fixthe shape. Alternatively, the powder material can be made plastic bymixing with water or with an organic binder and extruded into anydesired shape and sintered or fired into that shape. In anotheralternative method, the catalyst can be made into a liquid paint or inkwhich can be used to coat a support or walls, such as muffler walls, andthe like. After the coating is complete, the catalytic paint or ink canbe dried and fired to fix the coating to the underlying support or wall.If an extremely thin layer, such as 10³ A, of the catalyst on aparticular support is required, the catalyst can be coated on thesupport by ionic sputtering.

The operating temperature ranges cited are particularly wide for thecatalyst. This results from the low, high and overlapping temperaturesover which specific examples of the catalyst of the present inventionare characterized by high electronic and ionic conductivities whichcontribute to the electrical conductivity. This desired feature permitsoptimization with regard to chemical and thermal stability for aspecific application.

The following examples are included to further illustrate the presentinvention and are not intended as limitations thereof.

EXAMPLE 1

An aqueous mixture of 6 moles of a lanthanum nitrate (La(NO₃)₃) and 6moles of cobalt nitrate (Co(NO₃)₂) is prepared from the correspondinglanthanum and cobalt nitrate salts. The solution is boiled andevaporated to dryness; the resulting residue is calcined in air for 7hours at 1200° C. to produce a fine powder having the followingempirical formula:

    LaCoO.sub.3.

a substantially identical catalyst is prepared by employing thecorresponding chloride salts in place of the nitrate salts in the aboveprocedure.

The above powdered catalyst is moistened with distilled water to preparea thick paste which is extruded into macaroni-shaped tubes. The tubesare air-dried for 48 hours and then fired at 1300° C. for 3 hours.

EXAMPLE 2

An aqueous solution containing 1 mole of barium chloride, 9 moles ofyttrium chloride and 10 moles of titanium chloride was prepared bydissolving the corresponding chloride salts in 100 liters of water. Theresulting aqueous solution was evaporated to dryness under a vacuum. Theresulting residue was calcined for 10 hours at 1200° C. under anatmosphere of air to produce a fine powder having the followingempirical formula:

    Ba.sub.0.1 Y.sub.0.9 TiO.sub.3.

the resulting powder was pressed into cylindrical pellets measuring 5millimeters by 2.5 centimeters in a hydraulic press at 10,000 psi andsintered at 1300° C.

EXAMPLE 3

A catalyst is made by the procedure of Example 1 by calcining under aninert argon gas atmosphere to produce a fine ceramic powder having thefollowing empirical formula:

    LaCoO.sub.(3.sub.-m)

wherein m has a value between 0 and 0.11.

EXAMPLE 4

A catalyst is made by the procedure of Example 1 employing 6 moles ofchromium nitrate in place of cobalt nitrate and calcining under a 100%oxygen gas atmosphere to produce a ceramic powder having the followingempirical formula:

    LaCrO.sub.(3.sub.+m)

wherein m has a value between 0 and 0.11.

EXAMPLE 5

An aqueous solution containing 3 moles of calcium bromide, 7 moles of amixture of cerium nitrate, neodymium nitrate, and lanthanum nitrate, and10 moles of nickel chloride are prepared by heating 75 liters ofdistilled water to boiling and adding the above corresponding salts ofcalcium, cerium, neodymium, lanthanum and nickel. The solution is boiledto dryness and the residue is calcined under a nitrogen gas atmospherefor 5 hours at 1150° C. to produce a crusty residue of the empiricalformula:

    Ca.sub.0.3 (CeNdLa).sub.0.7 NiO.sub.(3.sub.+m),

wherein m is a value between 0 and 0.11, which powdered on scraping. Theresulting powder is moistened with water to produce a thin paste.Neutral alumina pellets are rolled through the paste to coat thecatalyst thereon. The coated pellets are air-dried and fired at 1250° C.for 3 hours to bond the catalyst coating to the pellets.

EXAMPLE 6

A 100 liter aqueous solution containing 0.1 mole of calcium chloride,9.9 moles of a mixture of rare earth nitrates wherein the rare earthelements have atomic numbers from 57 to 71, and 10 moles of a 50:50mixture of chromium and manganese nitrate. The resulting aqueoussolution is boiled to dryness at 1 mm. of Hg vacuum and the resultingresidue is calcined for 8 hours at 800° C. under an atmosphere of air at2 atmospheres pressure. The resulting catalyst powder is pressed intospherical shapes measuring 5 millimeters in diameter and sintered at1000° C.

EXAMPLE 7

To 10 liters of water, there are added 0.29 mole of strontium chloride,0.70 mole of ytterbium chloride, 0.01 mole of lutetium chloride, and 1mole of vanadium chloride. The resulting mixture is heated and stirredto form a solution which is filtered and evaporated to dryness at roomtemperature under vacuum to form a dry residue. The residue is calcinedto 1050° C. for 8 hours under an atmosphere of air to produce a ceramicpowder.

The powder is moistened and pressed into flat sheets 2.5 millimetersthick; the pressed sheets are dried and fired at 1200° C. for 9 hours toform ceramic sheets which are thereafter broken into 1 centimeterflakes.

EXAMPLE 8

A mixture of 14.2 g. of magnesium acetate, 23.3 g. of barium sulfate,229.0 g. of lanthanum carbonate, 57.4 g. of erbium oxide, and 79.8 g. offerric oxide is milled into a fine powder. The resulting powder ispressed into rods 1 cm. in diameter and fired at 1200° C. under anatmosphere of nitrogen for 15 hours to produce catalytic ceramic rods.

The rods are broken up to form pellets measuring about 1 cm. on a side.

EXAMPLE 9

A mixture of 283 g. of lanthanum sulfate, 50 g. of chromic anhydride,and 77.5 g. of cobalt sulfate is milled to a fine powder. The powder ismoistened with a 7:3:30 mixture of water, glycerine and gum arabic toform a paint. The catalytic paint is coated on fired alumina plates andallowed to air-dry. The coated plates are then fired in air at 1100° C.for 36 hours to produce a ceramic catalytic plate.

EXAMPLE 10

An air stream containing 10 ppm carbon monoxide was heated to 200° C.and passed through a bed of a ceramic, mixed oxide nonstoichiometric,electrically neutral catalyst having the following empirical formula:

    Sr.sub.0.1 La.sub.0.9 CoO.sub.(3.sub.+-m)

wherein m has a value of from 0 to about 0.11. The catalyst bed measured1 centimeter in diameter and 10 centimeters in length and the velocityof the gas was 10 centimeters per second. The effluent gas was cooled toroom temperature and analyzed with gas liquid chromatography using athermal conductivity detection cell and a 30 foot column containingmolecular sieve which showed that the resulting air stream had less than5 parts ppm. of carbon monoxide and no nitric oxide. Substantiallysimilar results will be obtained by employing the other catalystsdescribed above in the temperature ranges where they are characterizedby having significant electrical conductivities.

Substantially the same results are obtained by employing a ceramic,mixed oxide, nonstoichiometric, electrically neutral catalyst of thefollowing formula:

    SrLa.sub.9 (CrO.sub.(3.sub.+-m)).sub.10

wherein m has a value of from 0 to about 0.11 and heating the air streamto 400° C.

EXAMPLE 11

An air stream containing 5% carbon monoxide and 10% water is passedthrough a bed of sintered pellets of a ceramic catalyst of the presentinvention having the following empirical formula:

    CaLa.sub.9 (NiO.sub.(3.sub.+-m)).sub.10

wherein m has a value between 0 and 0.11. The catalyst bed measures 1centimeter by 15 centimeters and is heated to about 350° C. The velocityfor the gas stream is 12 centimeters per second. The effluent gascontains less than 10 parts per million (ppm.) carbon monoxide andvirtually no nitric oxide.

EXAMPLE 12

An air stream containing 15% sulfur dioxide is passed through a bed ofceramic flakes of a ceramic nonstoichiometric catalyst having thefollowing empirical formula:

    YFiO.sub.(3.sub.+-m)

wherein m has a value of from 0 to about 0.11. The catalyst is preparedaccording to the procedure of Example 7. The catalytic bed measures 1centimeter in diameter and 12 centimeters in length and is heated to250° C. The effluent gas contains sulfur trioxide and is substantiallyfree of sulfur dioxide, and nitric oxide.

Substantially identical results are obtained when the catalyst ismaintained at a temperature between 300° C. and 450° C.

EXAMPLE 13

An air stream containing 4% hydrogen sulfide is passed through a bed ofa powdered ceramic nonstoichiometric catalyst of the following empiricalformula:

    Sr.sub..2 Ce.sub..8 CoO.sub.3.sub.+-m

wherein m has a value of from 0 to about .11. The catalyst can beprepared according to the procedure of Examples 1, 3, or 4. The gasstream is heated to 400° C. and the velocity of 20 centimeters persecond is maintained through the catalytic chamber which measures 0.8centimeters in diameter by 12.5 centimeters in length. The effluent gascontains sulfur trioxide and water and is substantially free of hydrogendisulfide and nitric oxide.

EXAMPLE 14

An air stream containing 5% ammonia is heated to 1000° C. and passedthrough a bed of a powdered, ceramic, nonstoichiometric catalyst of thefollowing empirical formula:

    Y.sub..2 La.sub..2 Ce.sub..2 Nd.sub..2 Sm.sub..2 NiO.sub.3.sub.-m

wherein m has a value between 0 and 0.11. The catalyst chamber measures10 centimeters in diameter by 1 meter and the velocity of the gas streamis maintained at 7.5 liters per second. The effluent gas stream isessentially free of ammonia and contains nitric oxide and water as themajor by-products.

An air stream containing 10% nitric oxide and heated to 225° C. wasutilized in place of the air stream containing ammonia in the abovemethod to yield an effluent gas stream containing less than 0.2% nitricoxide; the major product is nitrogen dioxide.

EXAMPLE 15

A gas stream containing equal portions of carbon monoxide and nitricoxide is passed through a catalytic chamber containing a bed of sinteredceramic nonstoichiometric catalytic pellets of the following empiricalformula:

    Mg.sub..3 Ho.sub..7 TiO.sub.3.sub.-m

wherein m has a value of from 0 to about 0.11. The gas stream ismaintained at 650° C. and passed through the catalytic chamber at asufficient space velocity to insure that the reactants are in thecatalytic chamber for approximately 1/10 of one second. The effluent gasis substantially free of carbon monoxide and nitric oxide and consistsessentially of carbon dioxide and nitrogen gas.

The above carbon monoxide nitric oxide gas stream is replaced with anair stream containing 1% carbon monoxide, 1% nitric oxide and 2% water.The effluent gas stream is substantially free of carbon monoxide,contains less than 10 parts per million nitric oxide and consistsessentially of atmospheric gases and carbon dioxide.

EXAMPLE 16

The burners of a steam turbine generating plant boiler are operated withnatural gas fuel so that the maximum flame temperature is 1500° C. Thenatural gas is burned in an oxygen deficiency to yield a combustion gascontaining carbon monoxide, atmospheric nitrogen, water vapor, andcarbon dioxide. The combustion gas also contains some nitric oxide andhydrocarbons. The ratio of CO₂ to CO in the gas is adjusted to a ratioof 95:5 by controlling the air supply. The combustion gas, after beingemployed in producing high-pressure steam, is combined with a 1% excessof atmospheric oxygen at a temperature of about 600° C. and passedthrough a catalytic chamber containing the catalyst of Example 1 at aspace velocity sufficient to insure that the combustion gas is incontact with the catalyst for about 0.10 second to yield a stack gaswhich is substantially free of nitric oxide and carbon monoxide andcontains principally carbon dioxide, water and atmospheric nitrogen.When the fuel is burned with a greater oxygen deficiency to yield a CO₂to CO ratio of 90 to 10, an even cleaner stack gas with respect tonitric oxide is obtained.

With increasing temperatures for the initial combustion stage,increasing portions of nitric oxide will be emitted in the combustiongases. The following table shows the parts per million of nitric oxideprpesent in the combustion gas at various combustion temperatures forthe first stage of combustion.

                  TABLE                                                           ______________________________________                                        Combustion     Parts per million of nitric                                    Temperature    oxide in combustion gas                                        ______________________________________                                        1500° C.   150 ppm                                                     1400° C.    32 ppm                                                     1300° C.    0.9 ppm                                                    1200° C.    0.09 ppm                                                   ______________________________________                                    

The above method provides a method for public utilities to produce powerwith fossil fuels and yet emit a stack gas containing a minimal amountof nitric oxide, carbon monoxide, and unburned fuel. At present, mostpublic utility plants producing energy from fossil fuel emit an exhaustgas containing from about 200 to about 1500 ppm of nitric oxide. In manyareas of the country, such as Los Angeles, public utilities are a majorcontributor, up to 10%, of the nitric oxide in the air.

The above method can be employed utilizing natural gas, oil fuel, orcoal with substantially the same results.

EXAMPLE 17

A carburetor and the timing of a reciprocal engine, which burns agasoline containing 0.5 gm of lead/gallon, is adjusted so that theengine, when operating in its normal operating temperature, burns aslightly rich fuel-air mixture having about a 1% oxygen deficiency. Theexhaust or combustion gas from the cylinders of the reciprocal engineare vented through manifolds and exhaust pipes into a mixing chamberwhererin the combustion gases are mixed with a slight excess ofatmospheric oxygen, that is a 1% excess of the stoichiometric amount ofoxygen needed to fully burn the carbonaceous material, i.e., unburnedand partially burned gasoline and carbon monoxide, present in thecombustion gases. The resulting mixture is then vented into a catalyticmuffler containing an alumina honeycomb matrix coated with a catalysthaving the following empirical formula:

    Sr.sub..1 La.sub..9 CoO.sub.3.sub.+-m

wherein m has a value of from 0 to about 0.11. The catalytic chamber isdesigned such that the exhaust gas will travel through the muffler inabout 0.1 second when the engine is working at its maximum revolutionrate. The combustion gas has a temperature of above 300° C. when itcomes in contact with the catalyst. The resulting effluent gas willcontain less than 10² parts per million of nitric oxide, less than 250parts per million of carbon monoxide, and will be substantially free ofgasoline or partially burned gasoline.

EXAMPLE 18

The combustion gases from a steam turbine generating plant burningfossil fuels in a 2.1% oxygen deficiency contains carbon dioxide, carbonmonoxide, sulfur, hydrogen sulfide, nitric oxide, atmospheric nitrogen,water vapor, and unburned or partially burned fossil fuel. Thecombustion gas, which is heated to about 700° C., is passed through acatalytic chamber containing the catalyst of Example 6 at such a ratesuch that the catalyst and all the combustion gas come into contact. Thegaseous pollutants in the resulting effluent gas are substantiallyreduced. In particular, the amount of carbon monoxide and nitric oxidecontained therein are substantially reduced because the nitric oxide andwater vapor have oxidized the carbon monoxide to carbon dioxide. Theresulting effluent gas which has now cooled down to about 100° C. andthe sulfur are collected by electrostatic precipitation. The resultinggas is heated to approximately 250° C. and combined with a 0.7% excessof the stoichiometric amount of oxygen needed to fully oxidize theunburned fuel, partially burned fuel, the sulfur-containing compoundsand the remaining carbon monoxide. The resulting air and effluent gasmixture is passed through a second catalytic chamber containing thecatalyst of Example 17. The second catalytic chamber is designed suchthat the gaseous mixture and the catalysts are in contact forapproximately 0.1 to 0.5 seconds. The resulting effluent gas consistsessentially of carbon dioxide, water vapor, atmospheric nitrogen andresidual oxygen. The effluent gas contains less than 10 parts permillion of nitric oxide, less than 5 parts per million of carbonmonoxide, and is substantially free of nitrogen dioxide and unburned orpartially burned fuel.

EXAMPLE 19

An air stream containing 3% carbon monoxide is passed through a bed ofceramic catalyst pellets of the following empirical formula:

    Sr.sub..1 La.sub..9 CoO.sub.3.sub.+-m

wherein m has a value of from 0 to about 0.11. The catalyst is preparedin accordance with the method described in Example 6. The catalystchamber measured 1 centimeter in diameter by 10 centimeters in length.The gas stream is maintained at a velocity of 10 centimeters per second.The process is repeated at different catalytic temperatures and theeffluent for each reaction temperature is chemically analyzed todetermine the conversion ratio of carbon monoxide to carbon dioxide. Theresults are shown in the following table.

                  TABLE                                                           ______________________________________                                        Gas Stream Temperature                                                                          Percentage Conversion                                       (Reaction Temperature)                                                                            of CO to CO.sub.2                                         ______________________________________                                        160° C.        20%                                                     200° C.        80%                                                     300° C.       100% (substantially)                                     ______________________________________                                    

We claim:
 1. A ceramic, nonstoichiometric, electrically neutral catalystfor treating exhaust gases from the combustion of fossil fuelscomprising a homogeneous mixed oxide compound having a unitary crystalstructure and having the following empirical formula:

    X.sub.n J.sub.(1.sub.-n) ZO.sub.(3.sub.-m)

wherein: X is an alkaline earth metal or mixture thereof; J is scandium,yttrium, a rare-earth element, or mixture thereof; Z comprisesmanganese, at least 0.01% of Z having an oxidation state other than +3;m is a number other than 0 having a value up to about 0.11; and n is anumber other than 0 having a value up to about 0.51.
 2. A ceramic,nonstoichiometric, electrically neutral catalyst for treating exhaustgases from the combustion of fossil fuels comprising a homogeneous mixedoxide compound having a unitary crystal structure and having thefollowing empirical formula:

    X.sub.n J.sub.(1.sub.-n) ZO.sub.(3.sub.-m)

wherein: X is an alkaline earth metal or mixture thereof; J is ascandium, yttrium, a rare-earth element, or mixture thereof; Z comprisesmanganese, at least 0.01% of Z having an oxidation state other than +3;m is a number other than 0 having a value up to about 0.11; and n is anumber having a value from 0 to about 0.51; said catalyst having anelectrical conductivity of about 0.1 to 1,000 ohms .sup.⁻¹ cm.sup.⁻¹ attemperatures of from about 200° C. to about 700° C.
 3. The ceramic,nonstoichiometric, electrically neutral catalyst as defined in claim 1,wherein:X is magnesium, calcium, strontium, barium, or a mixturethereof.